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      <div class="post-title">接口幂等</div>
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          2023-05-11 16:15:53
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                <span class="span--category">
                  <a href="/categories/%E5%90%8E%E7%AB%AF/" title="后端">
                    <b>#</b> 后端
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                <span class="span--category">
                  <a href="/categories/%E5%90%8E%E7%AB%AF/Spring-Cloud/" title="Spring Cloud">
                    <b>#</b> Spring Cloud
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      <div class="markdown-body">
        <p>幂等是一个数学与计算机科学概念。</p>
<ul>
<li>在数学中，幂等用函数表达式就是：<code>f(x)=f(f(x))</code>。比如求绝对值的函数，就是幂等<br>的，<code>abs(x)=abs(abs(x))</code>。</li>
<li>计算机科学中，幂等表示一次和多次请求某一个资源应该具有同样的副作用，或者说，多<br>次请求所产生的影响与一次请求执行的影响效果相同。</li>
</ul>
<h2 id="如何设计幂等"><a href="#如何设计幂等" class="headerlink" title="如何设计幂等"></a>如何设计幂等</h2><p>幂等处理的过程，说到底其实就是过滤一下己经收到的请求，当然，请求一定要有一个<br><code>全局唯一的 D 标记 </code>。然后，怎么判断请求是否之前收到过呢？把请求储存起来，收到<br>请求时，先查下存储记录，记录存在就返回上次的结果，不存在就处理请求。<br><img src="http://qiniu.c77544s.top/picgo/202305111620137.png" alt="image.png"></p>
<h2 id="几种实现方式"><a href="#几种实现方式" class="headerlink" title="几种实现方式"></a>几种实现方式</h2><h3 id="select（可省）-insert-主键-x2F-唯一索引冲突"><a href="#select（可省）-insert-主键-x2F-唯一索引冲突" class="headerlink" title="select（可省） + insert + 主键&#x2F;唯一索引冲突"></a>select（可省） + insert + 主键&#x2F;唯一索引冲突</h3><p>交易请求过来，我会先根据请求的<strong>唯一流水号</strong> <code>bizseq</code> 字段，先 <code>select</code> 一下数据库的流水表</p>
<ul>
<li>如果数据已经存在，就拦截是重复请求，直接返回成功：</li>
<li>如果数据不存在，就执行 <code>insert</code> 插入，如果 <code>insert</code> 成功，则直接返回成功，如果 <code>insert</code> 产生主键冲突异常，则捕获异常，接着直接返回成功。</li>
</ul>
<h3 id="状态机幂等"><a href="#状态机幂等" class="headerlink" title="状态机幂等"></a>状态机幂等</h3><p>很多业务表，都是有状态的，比如转账流水表，就会有 <code>0-待处理，1-处理中、2-成功、3-失败状态 </code>.转账流水更新的时候，都会涉及流水状态更新，即涉及状态机（即状态变更图）。</p>
<h3 id="防重表"><a href="#防重表" class="headerlink" title="防重表"></a>防重表</h3><p>第一种是建立在业务流水表上 <code>bizseq</code> 的唯一性上。很多时候，我们业务表唯一流水号希望后端系统生成，又或者我们希望防重功能与业务表分隔开来，这时候我们可以单独搞个防重表。当然防重表也是利用主键&#x2F;索引的唯一性，如果插入防重表冲突即直接返回成功，如果插入成功，即去处理请求。</p>
<h3 id="token-令牌"><a href="#token-令牌" class="headerlink" title="token 令牌"></a>token 令牌</h3><ol>
<li>客户端请求申请获取 token, 服务端生成 token 返回</li>
<li>客户端带着 token 请求，服务端校验 token，token 存在则判断为第一次请求，token 不存在则为重复请求。</li>
</ol>
<h3 id="悲观锁"><a href="#悲观锁" class="headerlink" title="悲观锁"></a>悲观锁</h3><p>比如 <code>select for update</code>，配合事务实现幂等，注意这种锁要使用主键查询<br><img src="http://qiniu.c77544s.top/picgo/202305111642701.png" alt="image.png"><br>一般不建议使用悲观锁实现幂等</p>
<h3 id="乐观锁"><a href="#乐观锁" class="headerlink" title="乐观锁"></a>乐观锁</h3><p>就是给表的加多一列 <code>version</code> 版本号，每次更新记录 <code>version</code> 都升级一下 <code>(version=version+1)</code>。具体流程就是先查出当前的版本号 <code>version</code>,然后去更新修改数据时，确认下是不是刚刚查出的版本号，如果是才执行更新。</p>
<h3 id="分布式锁"><a href="#分布式锁" class="headerlink" title="分布式锁"></a>分布式锁</h3><p>分布式锁实现幂等性的逻辑就是，请求过来时，先去尝试获得分布式锁，如果获得成功，就执行业务逻辑，反之获取失败的话，就舍弃请求直接返回成功。</p>
<ul>
<li>Redis 分布式锁，可以使用命令 <code>SET EX PX NX+唯一流水号</code> 实现，分布式锁的 <code>key</code> 必须为业务的唯一标识</li>
<li>Redis 执行设置 key 的动作时，要设置过期时间，这个过期时间不能太短，太短拦截不了重复请求，也不能设置太长，会占存储空间。</li>
</ul>

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        <ol class="toc"><li class="toc-item toc-level-2"><a class="toc-link" href="#%E5%A6%82%E4%BD%95%E8%AE%BE%E8%AE%A1%E5%B9%82%E7%AD%89"><span class="toc-text">如何设计幂等</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E5%87%A0%E7%A7%8D%E5%AE%9E%E7%8E%B0%E6%96%B9%E5%BC%8F"><span class="toc-text">几种实现方式</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#select%EF%BC%88%E5%8F%AF%E7%9C%81%EF%BC%89-insert-%E4%B8%BB%E9%94%AE-x2F-%E5%94%AF%E4%B8%80%E7%B4%A2%E5%BC%95%E5%86%B2%E7%AA%81"><span class="toc-text">select（可省） + insert + 主键&#x2F;唯一索引冲突</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#%E7%8A%B6%E6%80%81%E6%9C%BA%E5%B9%82%E7%AD%89"><span class="toc-text">状态机幂等</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#%E9%98%B2%E9%87%8D%E8%A1%A8"><span class="toc-text">防重表</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#token-%E4%BB%A4%E7%89%8C"><span class="toc-text">token 令牌</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#%E6%82%B2%E8%A7%82%E9%94%81"><span class="toc-text">悲观锁</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#%E4%B9%90%E8%A7%82%E9%94%81"><span class="toc-text">乐观锁</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#%E5%88%86%E5%B8%83%E5%BC%8F%E9%94%81"><span class="toc-text">分布式锁</span></a></li></ol></li></ol>
      
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